Since it was reported the fact that a complex of polyethylene oxide (PEO) or polypropylene oxide (PPO) with a lithium salt showed an ionic conductivity in a solid state at ambient temperature, researches for utilizing it have been actively performed. However, according to the report of Armand et al., a polymer electrolyte using the PEO as a matrix exhibits a low ionic conductivity of below 10−8 S/cm due to its high tendency to crystallize at ambient temperature, and accordingly it could not be utilized. Recently, a polymer electrolyte of gel type which is plasticized after adding an organic electrolyte solution into a polymer matrix such as PMMA, PAN or PVC has been developed by G. Freuillade, M. Watanabe, E. Ysushida and Q. Wixwat et at. In the above plasticized polymer electrolyte in gel form, a lithium salt dissolved in an organic solvent presents in the polymer matrix having a dipole moment. In addition it exhibits an ionic conductivity of no less than 10−3 S/cm under a condition that the ratio of organic solvent and the lithium salt is optimized, and therefore, it has been highlighted as a polymer electrolyte system having a highest possibility to be utilized. However, in order to fabricate the above polymer electrolyte in gel type, a drying process should be performed after a heating process at over 100° C. is performed. In addition, because the matrix exhibits high viscosity in a melted state, the practical assembling process becomes complex, and the fabrication cost is increased.
U.S. Pat. No. 4,830,939 discloses a method for preparing a UV-cured polymer electrolyte by mixing a polyethylene composition having at least one unsaturated functional groups with a liquid electrolyte and then plasticizing the resultant with a cross-linking polymerization using a UV curing method. Although the UV-cured polymer electrolyte prepared by the above method exhibits a high ionic conductivity, the oligomer used as a matrix has a low mean molecular weight of 300–400, and therefore, it has a disadvantage in that its flexibility is relatively low.
In addition, European Patent No. 0 638 950 A1 discloses a method for preparing a UV cured polymer electrolyte which is plasticized with a cross-linking polymerization of a composition in which poly(ethylene glycol)-diacrylate (PEGDA) and a liquid electrolyte are mixed. The mean molecular weight of the oligimer used in this method, that is PEGDA, is limited to be no more than 440. However, the UV-cured polymer electrolyte prepared by the above method is a glassy polymer which has a high brittleness and therefore can not be stretched, to result a failure of its utilization.